Meanwhile, a top US Navy official today (April 17) warned the Kremlin is deploying its ballistic nuclear missiles and attack submarines in numbers, range and aggression not seen since the Cold War.

"NATO is viewed as an existential threat to Russia, and in the post-Cold War period, the expansion of NATO eastward closer to Russia and our military capability they view as a very visceral threat to Russia," Admiral Mark Ferguson told CNN.

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BUILD-UP: Russia has been launching a new fleet of submarines in recent years

But he said: "The submarines that we're seeing are much more stealthy.

"We're seeing the Russians have more advanced weapons systems, missile systems that can attack land at long ranges, and we also see their operating proficiency is getting better as they range farther from home waters."

Moscow is adding or upgrading some 12 naval bases across the Arctic Circle – threatening the UK.